Understanding in-situ ozone production in Delhi, India

Delhi suffers from some of the poorest air quality in the world. While ambient NO₂ and particulate matter (PM) concentrations have received considerable attention, ozone (O₃) levels are often ignored. High levels of O₃ can adversely affect human health, leading to respiratory problems. With recent studies suggesting a reduction in PM in China is leading to increased surface-level O₃ production,¹ it is important to assess the chemical processes in Delhi's atmosphere. To identify the main chemical drivers of O₃ production, detailed analysis of its photochemical precursor species is required.

During May/June (pre-monsoon) and October (post-monsoon) 2018, concentrations of a large range of speciated VOCs (C₂-C₁₃), oxygenated VOCs (o-VOCs), NO_x, HONO, CO and O₃, and spectral radiometry, were continuously measured at an urban site in Old Delhi. During the pre-monsoon campaign, O₃ concentrations regularly breached recommended WHO 8-hour exposure limit of 50 ppbv, with daytime peaks in excess of 190 ppbv.

Observations from both campaigns reveal very high concentrations of aromatic and o-VOCs, known to play an important role in in-situ O₃ formation, with total BTEX (benzene, toluene, ethylbenzene, xylenes) exceeding 150 ppbv. We also compare the relative contributions of anthropogenic and biogenic VOCs to O₃ production, which highlights the importance of unusually high night-time levels of reactive monoterpenes in Delhi.

The sensitivity of O₃ formation to changes in VOC and NO_x concentrations, photolysis rates and HO₂ radical uptake to PM, is explored using an observationally constrained photochemical box model, incorporating the Master Chemical Mechanism (MCMv3.3.1, mcm.york.ac.uk) into the AtChem2 modelling toolkit.² The model was constrained to measurements of 47 VOCs, NO, CO, HONO, 34 photolysis rates, aerosol surface area, and meteorological observations. This work probes the chemical processes that drive in-situ O₃ production in Delhi's atmosphere, highlighting the importance of using improved speciation of VOCs over standard regulatory measurements, which will aid the development of pollution control strategies.